Self-cleaning surfaces using nanostructured titania (nTiO2) have been of significant recent academic and industrial interest, showing potential on glass surfaces providing antibacterial properties [1-3]. Research into TiO2 as a photocatalyst semiconductor originally began in the early 1970's with the pioneering work of Honda and Fujishima who investigated the splitting of water into oxygen and hydrogen using TiO2 irradiated by UV light [4]. Currently, TiO2 photocatalysis is actively used in the field of photodegradation of organic compounds, specifically in environmental decontamination of air [5] and water [6]. Although most photocatalytic self-cleaning coating research has focused on self-cleaning glass, [7] self-cleaning polymers for paints and coatings are of significant potential industrial and scientific importance. However, little work has been performed on the chemistry for the integration of nano titania (nTiO2) into polymers for self-cleaning coatings. As dirt and bacteria accumulate on almost every surface, nanocomposites that both strengthen the polymer, while providing self-cleaning behavior would be of significant interest.
Inorganic/organic hybrids are emerging materials for polymer coatings due to their extraordinary and unique combination of properties originating from the synergism between the inorganic nanoparticles and the polymer. Addition of a relatively small amount of the nanoparticles (e.g., less than 10 wt. %) dramatically changes the properties of the resulting polymer nanocomposite. As examples, nTiO2 was used as a radiopacifier in dental composites and bone cements, [8, 9] as a solid plasticizer of polyethylene oxide (PEO) for lithium batteries, [10, 11] as a dye in a conjugated polymer for photoelectrochemical[12] or photoconductive[13] agents, and as a photocatalyst in a photodegradable TiO2-polystyrene nanocomposite films [14].
Due to their extremely large surface-area/particle-size ratio, nanoparticles have a thermodynamic tendency to aggregate into clusters, reducing the resultant properties of the nanocomposite materials [15]. Many efforts have been taken in order to increase the nanoparticle dispersion and to enhance the filler-matrix interaction [16]. Increasing the dispersion of TiO2 nanoparticles into a PVC polymer matrix was shown to increase the photocatalytic degradation significantly [17, 18]. One approach is breaking down the agglomerated nanoparticles using a mechanical method such as ultrasonic irradiation, which has been explored for dispersion of SiO2, TiO2, and Al2O3 nanoparticles during the synthesis of inorganic/polymer nanocomposite materials [19-21]. However, this approach is restricted due to the limited interaction between the inorganic fillers and the organic matrix, compared with the very strong interaction between individual nanoparticles.
An improved approach, termed “grafting to” or the polymer approach is modifying the surface of the inorganic filler with covalent attachment of the polymer chains minimizing agglomeration, while strengthening the interaction between the nanofiller and the polymer matrix. In a separate approach, the “grafting from” or monomer approach, polymer chains are grown from a nanosurface providing potentially higher graft densities and better control of the molecular weight and polydispersity of the polymer chains [22-25].
It would therefore be advantageous to provide self-cleaning coatings which avoid the above-mentioned limitations.